1. Field of the Invention
The present invention relates to an apparatus and a method for radio transmission of user data, arranged in data words of a defined length, in time-division multiplex frames that comprise active time slots in which data are transmitted and inactive time slots in which no data are transmitted. The invention is suited particularly for application in digital cordless telephone systems.
2. Description of the Related Art
Since the beginning of the 1990s, there has existed in Europe the DECT (Digital Enhanced Cordless Telephone) standard for cordless telecommunication. This system has ranges of from less than 300 meters up to about 10 km, depending on the antenna and the environment, and is particularly suited for use inside buildings. Both speech signals and data signals can be transmitted.
The DECT air interface is based on a combination of FDMA (Frequency Division Multiple Access=frequency-division multiplexing) and TDMA (Time Division Multiple Access=time-division multiplexing). In a frequency range from 1880 MHz to 1900 MHz, ten carrier frequencies are available, with a respective channel spacing of 1728 kHz. As is described for example in David, Benkner, xe2x80x9cDigital Mobilfunksysteme,xe2x80x9d Stuttgart 1996, chapter 8.1, each carrier frequency is divided into 24 time slots. The first twelve channels are reserved for the downlink from a fixed station to a mobile part of a mobile telephone system, and the following twelve time slots are reserved for the uplink from the mobile part to the fixed station. The 24 time slots form a frame having a total duration of 10 ms. The duration of an individual time slot is thus 417 xcexcs, in which 320 bits of user data, as well as 104 bits of synchronization data, signaling data and error correction bits, can be transmitted. The user data rate thus amounts to 32 kbit/s.
An alternative structure of a time-division multiplex frame is shown schematically in FIG. 2 of the accompanying drawings. The frame duration is again 10 ms. Between the uplink and downlink time slots, in which the data are transmitted from the fixed station to the mobile part or vice versa, there are inactive time slots in which no data are transmitted, respectively arranged between two active time slots. These inactive time slots (blind slots) allow a reprogramming of the frequency of the RF transceive module (frequency hopping) between two successive active time slots. In the present case, the duration of each active time slot is 833 xcexcs and the duration of each inactive time slot is 417 xcexcs, i.e., half of an active time slot. The active time slot has twice the duration of a time slot in the DECT system, and the inactive time slot has the same duration as a time slot in the DECT system. These longer active time slots can be required in order to ensure, despite narrowband frequency channels, a sufficient transmission capacity of, for example, 320 bits per time slot, corresponding to 32 kbit/s. The time duration from the beginning of an active time slot to the beginning of the subsequent active time slot is 1.25 ms. This alternative time-division multiplex frame structure can, for example, be applied in the US-American ISM (Industrial, Scientific, Medical) band, at a carrier frequency of 2.4 GHz.
Independent of the frame structure used, in each time-division multiplex system in a cordless telephone system, the continuous flow of information in both directions of transmission must be compressed into the comparatively narrow time slots. For this purpose, the speech data, coming, for example, from a fixed network line, are stored continuously in a memory, and are then converted, in what are known as bursts (i.e. transmissions with a significantly higher relative data rate), into the respectively selected time slot for radio transmission. In order to enable a qualitatively high-value telephone connection, the time delay that thereby occurs should be as small as possible.
The present invention is thus based on the object of proposing an apparatus and a method for radio transmission of user data, arranged in data words of a defined length, in time-division multiplex frames made up of active and inactive time slots, whereby, independent of the selected time slot, a minimal time delay occurs in the conversion of the user data.
According to the invention, the object is solved by means of an apparatus and a method for radio transmission whereby n (n=a whole number xe2x89xa71) data words of the user data are respectively stored during the time duration from the beginning of an active time slot to the beginning of the subsequent active time slot.
In this way, independent of the selected time slot, at the beginning of the respective time slot the storing of a data word into the memory is always completed exactly at the beginning of the respective time slot, so that the data can be converted into the time slot and transmitted with the smallest possible time delay.
The data word length can thereby be a byte, corresponding to eight bits of the user data. Preferably, five bytes of the user data can respectively be stored during the time from the beginning of an active time slot to the beginning of the subsequent time slot.
According to a preferred exemplary embodiment of the inventive apparatus and of the inventive method, the data rate with which the user data are temporarily stored in the memory is 32 kbit/s, and the rate at which data are read out from the memory is 576 kbit/s.
The time-division multiplex frame can thereby have a total duration of 10 ms, and can be divided into four active time slots having a respective duration of 833 xcexcs for the transmission of data between a fixed station and a mobile apparatus, four active time slots having a respective duration of 833 xcexcs for the transmission from the mobile apparatus to the fixed station, and eight inactive time slots having a respective duration of 417 xcexcs.
The quantity of data transmitted per time slot is preferably 320 bits of the user data. The carrier frequency of the radio transmission can be for example 2.4 GHz. The inventive apparatus and the inventive method can be used for the transmission of speech signals and data signals.